Several so-called anti-camcorder techniques have been developed recently for combating this illegal copying. In the subsequent description, the expression illegal copy is understood to imply a film copy made fraudulently by a camcorder or a camera. Likewise, the expression illegal viewer refers to a viewer watching an illegal copy. Conversely, a film copy which has been distributed legally is referred to as a legal copy. Legal viewer refers to a viewer watching a legal copy. These anti-camcorder techniques are essentially aimed at impairing the quality of the images captured by the camcorder by utilizing the differences between the human visual system and the camcorder's image capture system.
A first known anti-camcorder technique consists in temporally modulating the colour of pixels of the source image around their starting value in the source image. The modulation frequency is determined so that the modulation is invisible to the human eye but appears on the copy captured by the camcorder. If only the chrominance of the pixels is modulated, the modulation frequency is for example selected so as to be greater than the colour fusion frequency of the human eye, which is of the order of 20 Hz. If only the luminance of the pixels is modulated, the modulation frequency is selected so as to be greater than the flicker frequency, which is of the order of 60 Hz. The image pixels thus modulated can be arbitrary but are more generally selected to represent a so-called scrambling pattern on the images captured by the camcorder. This scrambling pattern is for example a message, a text or a symbol.
This temporal modulation technique is illustrated by FIGS. 1 and 2. FIG. 1 shows a source image representing a scene in which a character 10 is placed in front of a background 11 of different colour. This source image has to be protected against copying. The temporal modulation technique consists in modifying the colour of certain pixels of the image to reveal the scrambling pattern “COPY” on the images captured by the camcorder as illustrated in FIG. 2. With this aim, the source image of FIG. 1 is decomposed into n different component images which will be displayed successively. In the example of FIG. 2, the source image is decomposed into 2 component images. The colour-modulated pixels belong to the background 11 of the scene. In the first component image (left image), the colour of the pixels of the pattern is o equal to C+ and, in the second component image (right image), it is equal to C−. When these two component images are displayed successively with a display frequency greater than the colour fusion frequency, the human eye perceives a colour C which is the mean of the colours C+ and C−. The pattern is therefore invisible to the legal viewer.
FIG. 3 illustrates a way of defining the colours C+ and C− with respect to the colour C. In this example, it is considered that the colours C+ and C− have one and the same luminance and different chrominances. FIG. 3 represents the CIE xy chromaticity diagram generally associated with the CIE xyY colour space where Y represents the luminance and x and y represent the chrominance. In this diagram, the chrominances visible to the average human being lie in the horseshoe-shaped zone. This zone is conventionally called the gamut of human vision. Each colour of the space, defined by a point in the CIE xyY space, can also be defined by a colour vector (not represented) linking the origin of the xyY reference frame to the said point in space. The colours displayable by a video projector are contained inside a triangle T whose extremities correspond to the three primaries of the projector, namely red, green and blue. The bidimensional space formed by the CIE xy chromaticity diagram not being linear in relation to human vision, the points identifying the colours C+ and C− are defined in another space, the CIE XYZ space, which is recognized as being linear in relation to human vision. In the CIE XYZ space, the points identifying the colours C+ and C− are defined in such a way that they belong to the straight line passing through the point identifying the colour C, the two points representing the colours C+ and C− being equidistant from the point of the colour C. Of course, in order for these two colours C+ and C− to be displayable by the projector, they must be present inside the triangle T. Reasoning more generally in the CIEXYZ colour space, the colour vector associated with the colour C+ is defined to be symmetric, with respect to the colour vector associated with the colour C, with the colour vector associated with the colour C−. Moreover, to increase the amplitude of the modulation and maximize the effect on illegal copying, the remotest possible points C+ and C− are preferably selected.
According to this temporal modulation technique, the eye of the viewer integrates the two colours C+ and C− and then perceives the resulting colour, i.e. the colour C. The pattern is therefore invisible to the viewer. Nevertheless, when the integration of the two colours is interrupted by a blink of the eyes or when the two consecutive images are not projected at the same place on the retina because of micro-movements of the eyes, the colour of the pixel is no longer correctly reconstructed. The scrambling pattern may then be perceived by the legal viewer and disturb the latter inconveniently.
Another known anti-camcorder technique consists in using metameric colours to insert a scrambling pattern into the image sequence as described in the American patent application published under the number US2004/0081318. Two colours are said to be metameric when they are perceived as equivalent by the human visual system although having different spectra.
More precisely, the International Commission on Illumination (CIE) has defined three colorimetric functions, denoted x, yand z, for the response of the human visual system. These three functions represented in FIG. 4 are used to convert the spectrum of the light received by the eye into three values, denoted X, Y and Z respectively. These three values define the colour perceived by the eye. They can be calculated in the following manner:
      X    =                  ∑                  400          ⁢                                          ⁢          nm                          700          ⁢                                          ⁢          nm                    ⁢                                    E            ⁡                          (              λ              )                                ·                                    x              _                        ⁡                          (              λ              )                                ·          Δ                ⁢                                  ⁢        λ                  Y    =                  ∑                  400          ⁢                                          ⁢          nm                          700          ⁢                                          ⁢          nm                    ⁢                                    E            ⁡                          (              λ              )                                ·                                    y              _                        ⁡                          (              λ              )                                ·          Δ                ⁢                                  ⁢        λ                  Z    =                  ∑                  400          ⁢                                          ⁢          nm                          700          ⁢                                          ⁢          nm                    ⁢                                    E            ⁡                          (              λ              )                                ·                                    z              _                        ⁡                          (              λ              )                                ·          Δ                ⁢                                  ⁢        λ            
where E(λ) represents the energy of the illuminant.
It is therefore possible to calculate, for an illuminant whose spectrum is known, the values X, Y and Z. It should be noted that, in more complex models, the calculation of the values X, Y, Z can take account of the observer's age, the viewing angle and/or other parameters.
If a first illuminant whose spectrum is S1 is considered, the colour C1 perceived by the viewer is defined by the values X1, Y1 and Z1 such that:
            X      1        =                  ∑                  400          ⁢                                          ⁢          nm                          700          ⁢                                          ⁢          nm                    ⁢                                                  S              1                        ⁡                          (              λ              )                                ·                                    x              _                        ⁡                          (              λ              )                                ·          Δ                ⁢                                  ⁢        λ                        Y      1        =                  ∑                  400          ⁢                                          ⁢          nm                          700          ⁢                                          ⁢          nm                    ⁢                                                  S              1                        ⁡                          (              λ              )                                ·                                    y              _                        ⁡                          (              λ              )                                ·          Δ                ⁢                                  ⁢        λ                        Z      1        =                  ∑                  400          ⁢                                          ⁢          nm                          700          ⁢                                          ⁢          nm                    ⁢                                                  S              1                        ⁡                          (              λ              )                                ·                                    z              _                        ⁡                          (              λ              )                                ·          Δ                ⁢                                  ⁢        λ            
Likewise, if a second illuminant whose spectrum is S2 different from S1 is considered, the colour C2 perceived by the viewer is defined by the values X2, Y2 and Z2 such that:
            X      2        =                  ∑                  400          ⁢                                          ⁢          nm                          700          ⁢                                          ⁢          nm                    ⁢                                                  S              2                        ⁡                          (              λ              )                                ·                                    x              _                        ⁡                          (              λ              )                                ·          Δ                ⁢                                  ⁢        λ                        Y      2        =                  ∑                  400          ⁢                                          ⁢          nm                          700          ⁢                                          ⁢          nm                    ⁢                                                  S              2                        ⁡                          (              λ              )                                ·                                    y              _                        ⁡                          (              λ              )                                ·          Δ                ⁢                                  ⁢        λ                        Z      2        =                  ∑                  400          ⁢                                          ⁢          nm                          700          ⁢                                          ⁢          nm                    ⁢                                                  S              2                        ⁡                          (              λ              )                                ·                                    z              _                        ⁡                          (              λ              )                                ·          Δ                ⁢                                  ⁢        λ            
The human eve perceives an equivalent colour for C1 and C2 if
            C      1        ≡          C      2        ⇔      {                                                      X              1                        =                          X              2                                                                                      Y              1                        =                          Y              2                                                                                      Z              1                        =                          Z              2                                          
The colours C1 and C2 are said to be metameric if, perceived in an equivalent manner by the human eye, they have different spectra S1≠S2. C1 and C2 are said to correspond to one and the same visual colour since the values X1, Y1 and Z1 which visually define the colour C1 are identical to the values X2, Y2 and Z2 which visually define the colour C2. By extension, the spectra S1 and S2 are said to be metameric.
It is possible to produce metameric colours easily by using projection systems with four or more primaries. With these systems, the colour C1 is for example produced by using only three primaries (the colour component corresponding to the fourth primary is then zero) and the colour C2 is produced by using the four primaries (the colour component corresponding to the fourth primary is nonzero).
Referring again to the image of FIG. 1 and as illustrated in FIG. 5, this metamerism technique consists, within the framework of an anti-camcorder application, in employing the colour C1 (the spectrum S1) for pixels of the background which are intended to form the anti-scrambling pattern delimited by the dots, and the colour C2 (the spectrum S2) for the other pixels of the background 11. In this figure, the dots define the limit between that zone of the background 11 displayed with the colour C1 and that zone of the background displayed with the colour is C2.
The main difficulty in this technique is to reproduce metameric colours C1 and C2 which are perceived as equivalent by the human eye on the basis of different combinations of the primaries of the projection system. If these metameric colours are not perceived in a sufficiently equivalent manner, the scrambling pattern is then perceived by the legal viewer and disturbs the latter inconveniently.
When the scrambling pattern thus risks being perceived, even slightly, by the legal viewer, this defect is exacerbated by the movements of the objects of the scene with respect to the scrambling pattern, which, for its part, generally remains fixed in the successive images. To avoid this exacerbation in the case of “watermarks” inserted into the image, document WO2002/023905 teaches that the pattern be displaced with one of the objects of the scene, that is to say that the pattern be inserted into an object of the scene. Such an arrangement avoids the exacerbation of the abovementioned defects, but does not remove them since the pattern continues to be perceived in the moving object by the legal viewer.